Post by Porcell
So instead of people posting their guesses: (Source
www.wowwiki.com/Crusader_Aura/Mount speed modifiers increase multiplicatively, with each modifier increasing the players mounted speed (which is "100% + Mount Speed") by "100% + Modifier". Non-stacking multipliers are the only exception to this formula, in which case the largest possible modifier is only applied (either the product of the stacking modifiers or the non-stacking modifier itself).
Mount Type: Mounted Speed, Riding Crop, Crusader Aura
Normal Mount: 160%, 176%, 192%
Epic Mount: 200%, 220%, 240%
Epic Flying Mount: 380%, 418%, 456%
Swift Nether Drake/Al'ar: 410%, 451%, 492%
Normal running speed is considered 100% speed. An epic land mount adds 100%, making you move twice as fast or 200%. A riding crop increases your speed by 10%. 10% of 200% is an additional 20%, so a total of 220% running speed. An epic flying mount adds 280%, so you go 380% of running speed. An additional 10% would be 38% more, so a total of 418% of normal running speed.
The maximum increase from items is 10%, so all you need is a riding crop. A riding crop and a carrot does not stack. Additionally, the riding crop does not stack with Crusader Aura.